Optical sensor for detecting and localizing events

ABSTRACT

A system and method for a structure monitoring and locating a disturbance event is disclosed. The system includes a compact transceiver chip sending optical signals in three optical fibers that encompass the monitored structure appropriately. The system contains a sequence of loops, wherein the first and the second fiber forming the loop clockwise, while the third fiber is winded along the same loop counterclockwise. A set of two detectors registers the returning signals, and a time delay between those signals is calculated, which is indicative of the disturbance event location. The event location is determined with different sensitivity in different parts of the monitored structure depending on the density of fibers in these parts.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/466,128 filed May 14, 2009.

FIELD OF INVENTION

The invention is related to systems that provide information fordetecting and localizing events. In particular the invention relates tooptical fences with a distributed fiber interferometer sensor todetermine the location of a disturbance event along the object ofinterest and/or extensive perimeter area under observation.

BACKGROUND OF THE INVENTION

Fiber optic distributed sensing overcomes the inherent limitations oftraditional technologies, such as motion detectors, cameras,thermocouples and strain gauges, enabling monitoring solutions with newadvantages for the protection of people and critical assets, especiallywhen monitoring in inaccessible or inhospitable environments.

In a distributed sensor, the whole optical cable is the sensor itself.Typically, the fiber is integrated around (or into) a valuable asset(building, pipeline, cables, etc.). A single optical fiber can replacethousands of traditional single-point sensors, providing a significantreduction in installation, calibration, and maintenance costs. Inaddition, assets can now be monitored where previously this wasimpractical due to their size, complexity, location or environment.

The monitored structure in the present invention is any object or areaunder observation. The monitored structure can be the area enclosed intothe sensor system completely or partially. The monitored structure canbe an object, such as pipe, cable, fence, wall, etc. that is integratedinto the system by placing the optical fibers in a proximity to theobject or integrated into the object, for example installed into,wrapped by or winded around the object. A single monitored structure canbe comprised of different elements that are objects of, generally,different types, for example, monitored areas and monitored objects atthe same time.

The inherent distributed sensing nature of fiber optic sensors can beused to create unique forms of sensors for which, in general, there maybe no counterpart based on conventional sensor technologies. Opticalfiber is cheap, light, pliable, and immune to electromagneticinterference (EMI), which makes it a cost-effective, flexible and aninert sensor medium.

Most of the existing distributed sensors are based on DistributedScattering Sensing (Raman or Brillion), which has a limited sensitivityand immunity to various noises.

With modern low loss fibers and solid state laser diode sources, it hasbecome possible to develop systems having sensing fibers up to severaltens of kilometers in length. In one field of application, these fiberscan, for example, be placed under ground, carpets, imbedded in walls,under roads, or under turf. In such installations the sensors can beeffective in the detection of personnel, vehicles, animals, etc. into aprotected area of interest.

There is a need in well concealed fiber optic sensing systems to provideeconomic means for location of events, recognition and identification ofdisturbance events, and positive means for periodically and remotelyproof testing the integrity of the sensing fiber.

A few distributed optical fiber sensing systems based on a Sagnacinterferometer and a Mach-Zehnder interferometers have been previouslydeveloped. These systems depend on the interferometric detection ofphase differences between two optical signals whose relative phases havebeen shifted by changes in the optical properties of their respectivepaths caused by an external factor, such as, for example, acoustical ormechanical perturbation. The change in optical property of the fiberpath may be in the form of elongation, change in index of refraction,change in birefringence, or a combination of these or related effects.

While interferometer-based sensor systems have been developed with anumber of refinements, such systems have not been fully optimized foruse in applications that require simultaneous monitoring of differenttypes of objects.

Prior art, for example, U.S. Pat. No. 6,621,947 and U.S. Pat. No.6,778,717, both by E. E. Tapanes et al., discloses a principle (deviceand method) behind the counter-propagating signals detection indistributed fiber sensor. Optical signals are launched from a singlelight source into the waveguide (fence) and simultaneously detected by atwo separate photo-detectors. The difference between registered signalsallows detecting the place of the optical fence intrusion. Any parameterthat alters the fiber will affect both counter-propagating signals in asimilar fashion. Thus, the U.S. Pat. No. 7,519,242 by E. E. Tapanesshows the particular buried fiber configuration that is specificallysuited for detecting an intruder walking across the ground beneath whichfibers are buried.

However, it is desirable to have a distributed sensing system where theresponse to the potential perturbation can vary along the cable, beingadjusted to either the real system environment or a particular systemapplication. In other words, the real system implementation mightrequire different sensitivity at different areas being monitored,depending on, for example, different structure layouts, variousperturbation probabilities within different areas, or the differentnature of perturbations within different areas. The prior art does notprovide such desirable functionality.

Polarization phase shift variations are caused by dynamically varyingchanges in a signals polarization state versus the principalpolarization axis of the interferometer. As a result, receivedcounter-propagating signals can potentially interfere constructively ordestructively. Thus, it is also desirable, in addition, to have adistributed sensing system where polarization effects are intrinsicallymanaged by the system to dynamically address the variation ofpolarization states along the fiber sensor. Polarization managementtechniques for distributed fiber sensing have been disclosed in U.S.Pat. No. 7,499,176 by A. R. Adams and U.S. Pat. No. 7,499,177 by J.Katsifolis, as well as in U.S. Pat. No. 7,142,736 by J. S. Patel et al.Moreover, the U.S. Pat. No. 7,139,476 discloses the method ofdisturbance event detection/location by using the changes in the statesof polarization of counter propagating signals themselves. However, inall these configurations, the external polarization management wasapplied to the system with uniform perturbation response across theperimeter, as mentioned above.

There is a need for a distributed fiber sensing link with non-uniformperturbation response across the perimeter, which would drasticallyexpand the applicability of the system.

Although existing security application of distributed sensor systems arevaluable in detecting events over large areas, they are not alwayssufficiently sensitive, capable of dynamic adjustment/control,convenient in implementation, properly camouflaged or economical fordetermining the location of events of different nature at differentareas. Thus, several systems would be required to distinguish betweenperturbations, caused, for example, by events of different nature and/orperturbations that correspond to different environments encountered by asystem installation.

Operational pipelines are subject to complex, highly non-linear temporaland spatial processes that usually make it difficult to differentiatebetween faults and stochastic system behaviors. This makes detectingfailures/intrusions a challenging task, leading to integrating differenttypes of data that is remotely captured from several sources, such asproposed fiber-optic system, as well as pressure transient signals andflow (velocity) information. The various types of (high frequency) datacan be time synchronized. There is a need for a system capable ofdetecting small problems that might be precursors of catastrophicbursts, also enabling prompt detection and localization of larger leaksand malfunctioning equipment such as valves.

There is also a need for a technology to be used in earthquakecontinuous monitoring/early warning system. For example, when the systemhas detected a wave (P-wave-representing the warning of a futureimminent major earthquake), the visual and acoustic quake alarm can bestarted.

There is also a need to adapt existing monitoring system for underwateroperating conditions.

SUMMARY OF THE INVENTION

A system and method for structure monitoring and locating a disturbanceevent is disclosed. The system includes a compact transceiver chipsending optical signals in three optical fibers that encompass themonitored structure appropriately. The system includes a series ofloops, in which the first and the second fiber forming the loopclockwise, while the third fiber is winded along the same loopcounterclockwise, wherein the first and the second fibers have the sameoptical path. The fiber arrangement has different densities in differentparts of the monitored structure, such as, for example, critical placesin the structure may have a larger number of fiber loops surroundingthem. All fibers transmit signals in both directions: from thetransceiver to a returning point and back. A set of two detectorsregisters the returning signals, and the time delay between thosesignals is calculated using a digital signal processing (DSP) unit, thistime delay is indicative of the disturbance event location. Polarizationstates of the returning signals are controlled by the transceiver'sbuilt-in controllers. The event location is determined with differentsensitivity in different parts of the monitored structure depending onthe density of fibers at different locations.

The location P of the disturbance event along the first fiber isdetermined as P=(L−cΔt/n)/2, wherein L is a length of the first and thethird fibers, n is a fiber refractive index, c is a velocity of light,and Δt is the difference between the receiving times of the modifiedsignals.

In one embodiment the first and the second fiber are placed in onecable, which is shaped as a series of loops forming a chain-likestructure with each loop characterized by its waveguide length, loopperimeter shape, and number of windings.

In the preferred embodiment the transceiver formed as an integratedcomponent, including an input waveguide receiving light from the lightsource, a splitter which splits the input waveguide into a first and asecond waveguides, the first waveguide providing input for the firstlight beam into the first fiber, and the second waveguide providinginputs for the second light beam into the second fiber and the thirdlight beam into the third fiber; a first coupler splitting the secondwaveguide into a third and fourth waveguides; the third waveguide beingconnected to the second fiber and the fourth waveguide being connectedto the third fiber; a second coupler providing a first detectorwaveguide being connected to the first waveguide; the first detectorwaveguide leading the fourth signal to the first photo-detector; and athird coupler providing a second detector waveguide being connected tothe second waveguide, the second detector waveguide receiving the fifthand the sixth signals combined and leading it to the secondphoto-detector.

Yet another object of the present invention is a method for a structuremonitoring and locating a disturbance event. It includes the followingsteps: sending three optical signals via optical fibers from atransceiver to a returning point and then back; receiving the returnsignal by a set of photodetectors, measuring a time delay between thesignals and locating the disturbance event. The system sensitivity tothe disturbance event is different for different parts of the monitoredstructure. In the preferred embodiment the transceiver formed as anintegrated component.

A shape of the modified signals may be measured, and a type ofdisturbance is determined using a look-up table. Different kinds ofcorrelation and validation procedures may apply.

The system may be adapted for operating under water. In particular, itmay be used to detect acoustic pressure waves to study environment or tolocate and track maneuvering targets.

Another object of the present invention is a method for an objectmonitoring and locating a disturbance event. The method includes sendingthree optical signals via optical fibers from a transceiver to areturning point. The fibers form at least a first loop, wherein thefirst and the second fiber forming the loop clockwise, while the thirdfiber is winded along the same loop counterclockwise, wherein the firstand the second fibers have the same optical path. After passing thereturning point, the signals are received by a detector. The methodincludes determining a time delay between the signals and locating thedisturbance event.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conceptual view of the distributed interferometricsensing system with counter-propagating signals.

FIG. 2 illustrates one possible pattern for cables arrangement in thesensing fiber link.

FIG. 3 illustrates a loop-based layout for the sensor.

FIG. 4 shows a system embodiment with multiple loop segments along thesensing fiber link.

FIG. 5 illustrates a system embodiment with different loops layout alongthe fiber link. The sensor response to the perturbation at each sectionof the perimeter is different.

FIG. 6 shows a schematic example of a single sensing fiber link capableof detecting and locating various perturbations at different sections ofthe link A system response is customized for each of the section alongthe perimeter.

FIG. 7 shows spiral arrangement of fibers around a pipeline withdifferent density of fibers in different elements of the monitoredstructure (pipe).

FIG. 8 shows a system embodiment with counter-propagating signalspolarization management.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which the preferredembodiments of the invention are shown. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art.

The system is designed for structure monitoring and disturbancedetection and locating. The sensitivity to the potential perturbationcan vary along the fiber cable, depending on the requirements within aparticular area. Thus, the sensitivity of the system at particular areacan be optimized in terms of local structural layout, depend on howoften the perturbation occurs (event probability) or reflect thedifferent types of event within different areas (different types ofdamage, intrusions, etc.). The optimal sensitivity of the system isbased on combination of the foregoing factors that correspond to theeach particular monitoring area.

In the preferred system configuration, the fiber link is integrated intothe area with no visible physical barrier present. The system issuitable either for an underground arrangement or integrated into thestructure, providing a security (fence) and/or damage monitoring to arequired area. The technology is optimized for detection and location ofan event, caused by perturbation due to external intrusion orexternal/internal damage.

The fence includes the optical fiber cables with the counter-propagatingoptical signals having certain characteristics that can be modified oraffected by an external parameter caused by an event. The appliedstrain, vibration, acoustic emission, temperature transient, can causefiber elongation, change in index of refraction, birefringence, or acombination that, in turn, can modify the counter-propagating opticalsignals and indicate an event.

Continuous-wave optical signals are launched from a single light source1, FIG. 1, into the waveguide and simultaneously detected by twoseparate photo-detectors 2 and 3. (Pulsing of the optical signal may beemployed in some arrangements). The receiver detects the affectedcounter-propagating optical signals and determines the time delay ordifference between the modified counter-propagating optical signals inorder to determine the location of the event.

A returning point 4 provides for mixing and reversing the sent signals.It operates in both directions and has three input-outputs 5, 6, and 7connected to the first, second and third fibers respectively.

Any mentioned sensed parameter that alters the fiber will affect bothcounter-propagating signals in a similar fashion. However, each affectedcounter-propagating signals continues traveling the respective remainingportion of the waveguide loop to their respective photo-detector. Thiswill produce a resultant time delay or time difference between thedetected signals. The time delay is directly proportional to thelocation of the sensed event along the waveguide length. Therefore, ifthe time delay or difference is detected and measured, the location ofthe event can be determined, quantified and identified.

The receiver detects the interference pattern. Upon the disturbance aparameter of light passing through one of the waveguides is altered withrespect to the same parameter of the light passing through the otherwaveguide, and thereby it changes the interference pattern detected bythe detector.

An indication of the disturbance is provided by detecting the change inthe interference pattern due to the change in the waveguide parameter,where the light passing through one of the output waveguide is alteredwith respect to another.

In addition, in certain system configurations a non-sensitive fiberoptic delay line can be integrated at either or both ends in order toadd additional delay between the transmitted counter-propagatingsignals.

The system does not require signal averaging, and it determines thelocation of events via the time delay measurement betweencounter-propagating optical signals affected by the same disturbance.

Schematic set-up of the system, utilizing a Mach-Zehnder (MZ)interferometric sensing technique is shown in the FIG. 1.

This generalized embodiment illustrates the distributed sensing element,having Cable I and Cable II as the MZ interferometer arms that includesome arbitrary lengths of ‘insensitive’ lengths LI₀ and LII₁. While notplaying an active part in perturbation detection, these fiber segmentsprovide an additional optical delay between the transmittedcounter-propagating signals. This can substantially facilitate thepractical implementation of the system. The ‘active’ part of the fibersensor consists of segment LI₁ and LI₂ see FIG. 1. The perturbation canoccur at the point D, anywhere along this active part of the sensor,producing two identical perturbation of the signal, propagating inopposite directions: from D to C and from D to E, as shown in FIG. 1.The total optical fiber link with total distance:

L=LI ₀ +LI ₁ +LI ₂ +LII  (1)

The goal is to detect and locate a disturbance at point D, anywherealong the sensitive section of the fiber link The measurable parameteris the Δt—the difference in time of arrival of each of these signals atpoints C and E, measured by (balanced) photodiodes at the transceiverchip, see FIG. 1.

$\begin{matrix}{{\Delta \; t} = {{N\frac{\left( {{LI}_{2} + {LII}} \right) - \left( {{LI}_{1} + {LI}_{0}} \right)}{c}} = {N\frac{L - {2\left( {{LI}_{0} + {LI}_{1}} \right)}}{c}}}} & (2)\end{matrix}$

where c is the speed of light in a vacuum and N is the effectiverefractive index of the optical fiber (waveguide).

The perturbation location from one side of the link (point C) is givenby:

$\begin{matrix}{{{P_{C}{LI}_{0}} + {LI}_{1}} = {\frac{L}{2} - \frac{\Delta \; {tc}}{2\; N}}} & (3)\end{matrix}$

The same perturbation location from the opposite side of the link (pointE) is given by:

$\begin{matrix}{{{P_{E}{LII}} + {LI}_{12}} = {\frac{L}{2} + \frac{\Delta \; {tc}}{2\; N}}} & \left( {3a} \right)\end{matrix}$

This result illustrates that only knowledge of the total fiber linklength L is required, and not the respective lengths of the varioussensitive and insensitive fiber segments in the system.

The total fiber link may include multiple cable segments having various,generally different, patterns of the fiber arrangements along the link.The information about the total length and individual segments can beeasily obtained at the design and installation stages of a system aswell as by use of an OTDR method after installation.

Two (balanced) detectors detect the shape of correspondedcounter-propagating signals traveling in respective directions. Thisshape of is characterized by a rise-time and a fall-time of the saidsignals, providing a means of calculating a time delay Δt between themat the detectors.

Once the total length is known and the time delay Δt is measured by thesystem, the location of the sensed event can be readily determined byequation (3). The time difference between the two modifiedcounter-propagating signals will determine the disturbance location.

Alternatively, a single detector could be utilized to detect both of thecounter-propagating signals so that the signal detector has asynchronized reference to determine the time difference and the lengthalong the cables where a disturbance has occurred.

The event identification and localization can be validated by off-linevalidation procedure, which may include:

Pre-testing the sensor installations in their ambient environment. Forexample, the natural excitation technique (NExT) for the modal analysisof sensor installation structures excited in operating environment canbe used. It provides the state of a linear dynamic system from a seriesof noisy measurements.

Lookup tables can be used to compare with the input perturbation values,validating the event by matching (correlating) it against a library ofvalid (or invalid) events. In particular, the correlation may beestimated using minimum mean square error (MMSE) technique, or otherstatistical approaches may apply.

The first embodiment of the proposed system is shown in FIG. 2. Thefirst and second waveguides can share the same cable while a separatesecond cable utilizes only one (third) waveguide. The two cables havepreferably symmetrical pattern.

These separate cables can be placed under the surface (e.g. carpet andsuch) or buried underground. Cable I and Cable II are arranged in aproposed configuration (pattern) with respect to one another, defining aperimeter region having a custom width that is optimized uponapplication. The width can be defined by particular structural elementunder observation (e.g. pipeline), or can be defined by a distance whichis traversed by a person intruding into the area. For example, thesufficient width can be such that an intruder traveling in a walking orrunning motion would not step over the fence area. In this case, formaximum effect the cables should be laid in a shallow trench for theentire length of the sensitive zone.

The system can also be used for monitoring and detecting various soildisplacements, for example the ones associated with the undergroundtunneling activity. In preferred configuration, the buried sensingcables are laid along the bottom of the trench, in a closely spacedproposed loop-based pattern that runs across the full width of thetrench.

It is essential that the pattern of one cable is opposite to that of theother cable, in order to be out of phase. The cables may touch as theycross over or being overlapped. The maximum sensitivity can be optimizedfor the application.

In the preferred configuration, an integrated transceiver 10 isconnected to the light source 1. The light is split by a splitter 11into two beams travelling along the first 12 and the second 13waveguides. A coupler 14 splits the second beam and directs it to thesecond and the third fiber. Couplers 15 and 16 (optionally tunable) areintegrated in a single LN (LiNbO₃) chip, which also includes pair ofbalanced diodes 2 and 3. These couplers lead the returning signals tothe photodetectors. The cables are terminated at either each end of theperimeter or at one end only if the perimeter is closed. The returningcoupler 4 can also be LN integrated or, alternatively, made of fusedsilica fibers. The integrated configuration leads to a very compact,reliable and flexible system where the operating point can be easilyadjusted by means of electro-optical and/or thermo-optical controls.

Optionally, the simplified version may be based on the terminatingpolished mirrors instead of a coupler, providing an intrusion alertwithout determination the event location in this case.

The protected area can be completely or partially enclosed by theperimeter to provide complete monitoring of the region of interest.Anybody attempting to gain access into the protected area will walk overthe preferably hidden cables, and the weight of the intruder will applya load to the two cables or, potentially, move the cables as theintruder walks over, under and/or along the cables. This will cause achange in parameter, such as a phase of the signal, which, in turn,changes the interference pattern when the modified signal recombineswith the counter-propagating modified signal.

Similarly, a change in parameter, such as a phase of the signal, can becaused by damage to the structure along the monitored perimeter, suchas, for example, pipeline or bridge, where the cables are installed.

The system can be integrated into a structure, such as fence, pipeline,bridge or building elements. The cables can be installed into existingsite/layout, integrated into the monitored structure itself or buriedunderground by excavating a trench. The removed soil can be used tobackfill the trench after laying the cables. Once installed, the cablesdo not require any particular care, such as maintaining very precisepattern uniformity.

All configurations have the advantage that the concealed cables aresensitive enough to detect even the slightest foot-fall, continuouslyand discretely, twenty-four hours a day for a very long period of time.Their performance is unaffected by changes in the local environment(rain, hail, temperature, electrical storms and magnetic loads). Noiseand vibration effects from background traffic can be screened out.Washouts do not disable the system and can be repaired.

The advantage of the proposed system is that the fibers cannot bedetected by metal detectors or emission measurements, since cables notrequire any metal components and do not emit electromagnetic radiation.The sensitivity of the detecting system and corresponded alarm level canbe arranged to suit the application/operational requirements. Thesensitivity of the system is weakly dependent on the fiber length up toseveral tens of kilometers without signal amplification. Signalamplification can be used for greater fiber length.

It is desirable to have a distributed sensing system where thesensitivity to the potential perturbation can vary along the cable,being adjusted to the actual system environment/application. In otherwords, the real system implementation might require a customized(optimized) system response to events that occur at different areas ofthe monitored system depending on, for example, different structurelayouts, various perturbation probabilities within different areas, ordifferent nature of perturbations within different areas.

Distributed Sensing

Yet another loop-based layout for the interferometric fence structurethat based on the three foregoing fibers is proposed. A singleloop-element (period) of such fiber sensor is schematically shown in theFIG. 3. As it can be seen from the FIG. 3, each fiber has a definedwinding direction with respect to other fibers.

Here the parameters of the fiber sensor, such as, for example, a loopshape α and β, as well as the number of windings within the loop γ canbe adjusted accordingly to a required application and optimal response Rto particular perturbation Fp:

R(t)=R{Fp(L,t),α,β,γ},  (4)

where t is the time and L is the location along the cable.

The approach can be further extended by using a ‘chain’ of loops of,generally, different α, β and γ. Thus, FIG. 4 illustrates the trenchfilled with such multiple loops, each made of the three interferometricfibers, respectively.

The sensitivity of the fence system will depend on amount of windings ineach loop as well as on a number/shape of the loops, and can be improvedat the expense of fence complexity and additional fiber cost.

As mentioned, it is desirable to have a variable detection/locationcapability along the perimeter area of the system. It can be realized byvarying the amount of cable installed at the specific location of theperimeter. Thus, an example of the system, where the different amount ofcable associated with different area (sensitivity), is shown in FIG. 5.Here all parameters of the loops layout can vary along the perimeter L.In this case, optimal response R_(i), to particular perturbationFp(L_(i),t) will be a function of the particular configuration of thefiber at particular area of the perimeter L_(i):

R _(i)(L _(i) ,t)=R{Fp(L _(i) ,t),α_(i)(L _(i)), β_(i)(L _(i)), γ_(i)(L_(i))}  (5)

FIG. 6 schematically illustrates the implementation of the single systemfor monitoring different types of objects, where different systemresponse at different segments of the perimeter is required (distributedsensitivity).

Alternatively, different responsivity can be assigned to different partsof the same object, as illustrated in FIG. 7 for a single cable, wherethe number of fiber loops and its shape is adjusted, accordingly tospecific geometry of the monitored element or application requirements.

The detecting of a modified signal might not be sufficient todistinguish a perturbation of interest from other incidentalperturbations. In a preferred embodiment the system involves theelectronic analysis to decompose the detected signal and further use thetraining algorithms based on prerecorded database of variousperturbations.

Prerecorded database of perturbations may include, but not limited to,different known patterns of predictable events such as simulatedintrusions and simulated damages associated with different segments ofthe perimeter.

It is also often desirable in intrusion detection security systems toknow when authorized persons have entered a specific area of theperimeter for legitimate purposes and to record their identity. Insystems providing surveillance of spaces on campuses or withinbuildings, it is regularly necessary for authorized persons to entermonitored areas without setting off an alarm.

Alternatively, more than one described system can be arranged into asecured perimeter, separated by a certain distance.

The proposed invention is of special interest for pipeline monitoring.FIG. 7 shows one embodiment of the fiber arrangement in this case. U.S.Pat. No. 6,644,848 by Clayton et al. discloses a sensor winding aroundthe pipe. In our invention two cables are winding around the pipe, inopposite directions. The loop density, shape and size may be differentin different elements of the pipeline as shown in FIG. 7 for one cable.

The proposed technology can be used for the pipe-line monitoringincluding the distributed damage sensing, security monitoring orcombination of both. Such fiber optic pipeline intrusion andleak-detection system can operate over large distances detecting,locating and classifying noises and vibrations in the vicinity of thecable, distinguishing among leaks, tampering, intrusions, digging,machinery and vehicles operating nearby. It can help in preventingillegal tapping, intrusions and leaks that cannot be detected byconventional flow metering.

The system may be adapted for underwater operation. The ability toinstall proposed sensor underwater brings new opportunities to for asmall-area monitoring, structural monitoring, and industrialapplications. While fiber sensor systems are well established on theground, the underwater applications remain quite limited by comparison.The underwater sensing of acoustic waves can be used in number ofapplications, including seismic monitoring, equipment monitoring andleak detection.

Another promising application for underwater distributed-sensitivitysensor is seismic monitoring for oil extraction from underwater fields.This is a better alternative to the underwater monitoring whichtypically involves a ship with a towed array of hydrophones as sensorsinvolving both large capital and operational costs.

Another application can of the invention is using a broadband (0.05-60kHz) distributed-sensitivity-sensor system capable of a long-termmeasurement and characterization of acoustic noise spectrum in deep sea,which is a powerful tool to study the underwater environment. The systemalso may serve as underwater sensor arrays to locate and trackmaneuvering targets and detect perturbations caused by acousticspressure waves.

Polarization Management

The remote disturbance might be any of various physical occurrences thataffect the waveguide on a scale that is comparable to the wavelength ofthe light. Slightest instances of changing physical pressure, motion orvibration and the like can change light propagation conditionssufficiently in an optical fiber or similar waveguide, to produce aneffect that might be discerned as a disturbance and used as a basis tolocalize the effect. Theoretically, when a disturbance affects bothbeams propagating over unequal path lengths to a detector, a phasevariation should arise at the detector on one of the two beams first,after a propagation delay from the point of the disturbance. Adifference between the two signals may persist between the time ofreception of the first signal to arrive along the shorter path, untilthe time of reception of the second signal to arrive along the longerpath. After the second signal arrives, the same phase variation thataffected the first signal affects the second one, theoretically equally.An interference summing node is effectively a phase comparator. The timespan is a function of the difference in distances from the detector tothe disturbance along the two paths. From the time difference andinformation as to whether the phase difference leads or lags, thedisturbance can be located to a point. This point can even be at themiddle of the loop, with the zero indicative time-difference in such acase.

The polarization attributes of the counter-propagating signals have tobe taken into account. Birefringence changes polarization alignment byinducing a phase difference between two orthogonal components of a lightsignal. Polarization phase shift variations arise in part because thereare dynamically varying changes to the polarization states of the lightsignals between the signals as they are launched, versus the principalpolarization axis of the interferometer at which the received signalscan potentially interfere constructively or destructively. Thus, thedifference varies as a function of the birefringent state of the fiberalong the two counter-propagation paths and the change in polarizationalignment can involve a phase difference of its own. The effect ofpolarization fading and polarization induced phase shift can be quitedetrimental, leading to system failure if precautions are not taken.

Similarly to interferometric adjustment, the polarization aspect of thesignal can be adjusted to provide an accurate location of the event.Ideally, the state of polarization for the two interfering beams shouldbe adjusted to be substantially parallel to each other before theinterference, to avoid the polarization-induced signal fading andinduced phase shift. Accordingly, the intensity criteria can be used asan input to a feedback control for adjusting one or more polarizationcontrollers to maximize the amplitude of the intensity signal, i.e., toachieve the greatest available span between maximum and minimum levelsof constructive and destructive interference. In other words, thefeedback controls to the polarization controller can be arranged to makethe depth of modulation of the interference signal as large as possible.

At standby condition is assumed when the system is prepared or primed todetect a disturbance. To obtain a maximum value and maximum swing inintensity, two conditions are addressed, namely:

(a) the polarization orientations of the two beams are aligned,

(b) the phase difference between the two beams is set to zero.

Similarly, in order to obtain a minimum value of intensity:

(a) the polarization of the two beams are aligned;

(b) the phase difference between the two beams is π radians.

At the standby condition, the polarization transformation functions ofthe two counter-propagating optical channels can be effectively balancedby active management of the polarization conditions. Thus, thepolarization mismatch for the counter-propagating optical signals, aswell as effective phase, will always be the same at the standbycondition. As a result, the location of the disturbance can be localizedmore accurately since the lead/lag time used to calculate the locationcan be determined dependably and more accurately.

The received signals can be combined in a polarization insensitive way,by controlling the polarization state of the input beams. In this way,the time difference of the intensity response for the twocounter-propagating optical signals can be correlated at the point ofdetection. The received signals are matched in a way that eliminates theinterference signal intensity variations resulting from polarizationconditions and thereby demonstrates the lead/lag time without carryforward errors and complications caused by polarization effects.

In the absence of a disturbance, the intensity of interfering beamstheoretically should be more or less constant due to a stable degree ofconstructive and destructive phase cancellation, i.e., interference, ofthe two more or less constant signals.

In a single LN chip, electro-optically adjusted birefringence is notsufficient to achieve effective polarization transformation sincemode-conversion between the orthogonal TE and TM components is alsorequired. Efficient 100% electro-optical conversion can be achieved byutilizing an off-diagonal element of the electro-optical tensor to causemixing between the orthogonal TE and TM modes (normally uncoupled).However, an electro-optical T

TM converter alone is also not capable of providing general polarizationtransformation.

We propose to integrate into the system transceiver chip a (variableefficiency) polarization conversion scheme where the two electricalfields are applied alternately along the interaction region. Forexample, if electrical field applied parallel to the X direction of LNchip, an off-diagonal element in the dielectric permittivity tensor isinduced via the electrooptic coefficient r₅₁=28×10¹² m/V. In suchscheme, short sections of birefringence tuning electrodes areperiodically interleaved between short sections of T

TM mode converter electrodes, and a large number of sections are used intotal. Although this arrangement may result in longer interactionlengths, it clearly permits independent control of T

TM mode conversion and birefringence tuning. In such orientation the TEand TM modes have significantly different propagation constants due tothe large birefringence of LN. The propagation constant of the TM modeis mainly determined by the ordinary refractive index of LN, whereas thepropagation constant of the TE mode is mainly determined by theextraordinary index. As shown in FIG. 8, the two polarizationcontrollers 20 and 21, each based on 3-sections (cells), integrated intothe LN X-cut, Y-propagation transceiver chip.

The middle section of converters I 20 and II 21 in the FIG. 9 has aconstant electrode period Λ, and electro-optic TE

TM mode conversion is obtained when a voltage Vc is applied to the modeconverter electrodes. In this case, the conversion is most efficient ata wavelength λ₀ defined by the phase-match condition:

Λ=λ₀ /|no−ne|  (6)

Here, no and ne denote the effective phase indexes of the TE and TMmodes and λ₀ is the vacuum wavelength, where mode converter operateswith maximum conversion efficiency under chosen Λ. At other wavelengths,this would not be the case; the contributions of adjacent mode converterfingers add slightly out of phase, resulting in a reduced overallconversion efficiency. The optical bandwidth of mode conversion isdetermined by the total interaction length L, i.e. the 3 dB bandwidth(FWHM) is given by

Δλ/λ₀=Λ/L=1/N,  (7)

where N is the number of electrode periods.

Polarization controllers 20 and 21 are used to control polarizationeffects in the counter-propagating optical signals by establishing andmaintaining polarization states of the interfering beams for each of thecounter-propagating light signals that are amenable to interference ofparallel polarization components of the respective beams. This can beaccomplished using feedback control so as to cause a polarizationcontroller to seek maximum peak to peak interference signal amplitude.This and other related polarization management techniques permit aprocessor coupled to the detector (and optionally coupled to provide thefeedback signal to the polarization controller) efficiently, easily andaccurately to calculate the location of the disturbance.

Polarization controllers 20 and 21 are readjusted such that the state ofpolarization of the light beams travel along cables are parallel to eachother before they interfere with each other at the detector, for theclockwise and the counter-clockwise propagating light signals,respectively, see FIG. 9.

The proposed fiber sensor is preferably realized using integratedplanar-wave technology, such as LiNbO₃ electro-optical circuit andincludes built-in polarization control elements to dynamically addressthe variation of polarization states along the fiber sensor. By thesemeans, the phase difference due to signals polarization misalignment canbe managed (minimized), enabling an accurate detection/localizing of theperturbation (event).

Proposed polarization control elements are built into the sameelectro-optical chip, making them an inherent part of the compacttransceiver chip. Using integrated electro-optical planar-wavetechnology enables effective adjustment of the counter-propagatingsignals by tuning coupling ratios of splitters and combiners, opticalphase adjustments and polarization management. High integration ofelectro-optical components within the same chip/package leads to a verynoise-proof, compact and cost-effective solution.

The proposed system can either be placed under the ground surface,making it invisible for a potential intruder, or integrated into thestructure itself. The technology can also help in detecting tunnels deepunder ground, and monitoring the burred fiber-cable for the telltalesoil displacements associated with tunneling activity.

Another application of the proposed system is to utilize the fiber(s)within an (existing) telecommunication cable as the distributed sensorelement to provide information related to a damage or intrusion anywherealong cable length. Such an arrangement would be most valuable forprotection of the cable facilities of power or telephone companiesagainst unauthorized intrusion or tampering. When deployed in thismanner, the sensor can detect direct contact, mechanical pressure, oracoustic signals.

Distributed optical fiber sensing links disclosed in invention can beeffectively implemented in a wide variety of applications. Withoutlimitation, the field of use can include railroads, roads, walls, gates,and bridges maintenance, as well as pipeline infrastructure,construction, fences of various types, petrochemical/nuclear-powerinfrastructures; earthquake monitoring. Any application that requiresthe detection, measurement and locating of a particular disturbancealong the particular section of the fiber link can benefit from theinvention. The invention would offer lower cost, unprecedentedversatility, improved operational and safety-related characteristicsover existing technologies.

The description of a preferred embodiment of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. Obviously, many modifications and variations will be apparentto practitioners skilled in this art. It is intended that the scope ofthe invention be defined by the following claims and their equivalents.

1. A system for monitoring an object and for locating a position of adisturbance event, comprising: a transceiver producing a first, a secondand a third optical signals entering a first, a second and a thirdoptical fibers; the first, second and third fibers forming at least afirst loop, wherein the first and the second fiber forming the loopclockwise, while the third fiber is winded along the same loopcounterclockwise; the first, second and third fibers having a returningpoint; the returning point being a coupler operating in both directions;the coupler having a first, a second, a third input-outputs; the first,second and third input-outputs being connected to the first, second andthird fibers respectively; the first, second and third signals becominga fourth, a fifth and a sixth optical signals after passing thereturning point; the fourth, fifth and sixth optical signals propagatingfrom the returning point towards the transceiver along the first, secondand third fibers respectively; the first, second and third fibers beingcapable of having some characteristic of the first, second, third,fourth, fifth and sixth signals modified or affected by the disturbanceevent; and detector means for detecting the modified fourth, fifth andsixth optical signals and for determining a difference between thereceiving times of the modified signals in order to locate thedisturbance event.
 2. The system of claim 1, wherein the same opticalpath for the first and the second fibers in at least the first loop isachieved by combining the first and the second fiber together into aloop fiber by a combiner prior to the loop and splitting them by asplitter after the loop.
 3. The system of claim 1, further comprisingthe first and the second fibers forming a first cable and the thirdfiber forming a second cable, the first cable being shaped as a seriesof loops forming a chain-like structure each loop characterized by awaveguide length, loop perimeter shape, and number of windings.
 4. Thesystem of claim 3, wherein the fiber length, loop perimeter shape, andnumber of windings of each loop is optimized to provide a requiredsensitivity in different parts of the monitored structure.
 5. The systemof claim 3, wherein the chain-like structure is positioned along theobject perimeter and has different density of fiber loops in differentplaces along the perimeter.
 6. The system of claim 5, wherein theperimeter M consists of sequential segments m₁, m₂, . . . m_(N), andeach segment of the perimeter corresponds to the segments of the firstfiber l₁, l₂, . . . l_(N), and the disturbance location along theperimeter is determined basing on the known disturbance event P alongthe first fiber.
 7. The system of claim 1, wherein the detector meanscomprises: a first and a second photodetectors; the first photodetectorreceiving the fourth signal, and the second photodetector receivingcombined the fifth and the sixth signals.
 8. The system of claim 7,wherein signals from the first and the second photodetectors areprocessed in a digital processing unit (DSP) recovering informationabout the location of the disturbance event.
 9. The system of claim 8,wherein the location P of the disturbance event along the first fiber isdetermined as P=(L−cΔt/n)/2, wherein L is a length of the first and thethird fibers, n is a fiber refractive index, c is a velocity of light,and Δt is the difference between the receiving times of the modifiedsignals.
 10. The system of claim 1, wherein relative shapes of themodified counter-propagating signals are measured to calculate thelocation of the disturbance, while type of disturbance is determinedusing a look-up table.
 11. The system of claim 10, wherein thedetermination of the type of disturbance is performed automatically inDSP unit using a validation procedure.
 12. The system of claim 10,wherein the type of disturbance is determined automatically using acorrelation procedure.
 13. The system of claim 1, adapted for operatingunder water.
 14. The system of claim 13, adapted to determine the systemperturbation caused by acoustic pressure waves.
 15. The system of claim1, further comprising: the transceiver formed as an integratedcomponent, including an input waveguide receiving light from a lightsource, a splitter which splits the input waveguide into a first and asecond waveguides, the first waveguide providing input for the firstlight beam into the first fiber, and the second waveguide providinginputs for the second light beam into the second fiber and the thirdlight beam into the third fiber; a first coupler splitting the secondwaveguide into a third and fourth waveguides; the third waveguide beingconnected to the second fiber and the fourth waveguide being connectedto the third fiber; a second coupler providing a first detectorwaveguide being connected to the first waveguide; the first detectorwaveguide leading the fourth signal to the first photodetector; and athird coupler providing a second detector waveguide being connected tothe second waveguide, the second detector waveguide receiving combinedthe fifth and the sixth signals and leading it to the secondphotodetector.
 16. The system of claim 15, further comprising a firstand a second polarization converters in the first and the fourthwaveguides to align polarization states of the received signals.
 17. Amethod for an object monitoring and locating a disturbance event,comprising: sending a first, a second and a third optical signals from atransceiver to a returning point; the first, second and third signalsentering a first, a second and a third optical fibers; the first, secondand third fibers encompassing the monitored object; the first, secondand third fibers forming at least a first loop, wherein the first andthe second fiber forming the loop clockwise, while the third fiber iswinded along the same loop counterclockwise; the returning point being acoupler operating in both directions; the coupler having a first, asecond, a third input-outputs; the first, second and third input-outputsbeing connected to the first, second and third fibers respectively; thefirst, second and third signal becoming a fourth, a fifth and a sixthoptical signals after passing the returning point; receiving the fourth,the fifth and the sixth optical signals by detector means; determining atime delay between the fourth, the fifth and the sixth signals; andlocating the disturbance event.
 18. The method of claim 17, furthercomprising combining the first and the second fibers in one cable, whichleads to the same value of a phase change in the light beams propagatingin these fibers.
 19. The method of claim 18, further comprising: shapingthe cable in a series of loops forming a chain-like structure each loopcharacterized by a waveguide length, loop shape and number of windings.20. The method of claim 19, wherein the waveguide length, loop shape,and number of windings of each loop is optimized to provide a requiredsensitivity in the event detection and locating.